For descriptive clarity and to avoid repeated references to the alternative use of apparatus of this invention it will be described in conjunction with the usual oil field operations related to tubular strings joined by serial threaded connections. It will be understood that such tubing strings are representative of any threadedly connected machine elements manageable by the apparatus.
Historically, pipe string make-up, in well strings, was a combination of individual actions that used a cat head to add pulling power to the torque arm of tongs. Torque was produced by a force couple consisting of pull on the torque arm, an opposite force on the pipe being manipulated, and the space between those force vectors. The tongs were suspended and counterbalanced, but they were otherwise man handled.
Power tongs have come into widespread use in recent years and can be defined as tongs with bodies in which the pipe gripping dies rotate relative to the tong frame and about the gripped pipe centerline. Early power tongs still had the torque arm, snubbed to rig structure, and corresponding side loads on pipe still resulted from applied torque. The lateral load represents an undesirable, and sometimes unacceptable, side load on the pipe being assembled or dismantled.
Tongs in general oil field use have achieved definitions as lead tongs which rotate a pipe section relative to earth and back-up tongs which hold a pipe section stationary relative to earth. In operation, the relative rotation between the two tongs provide the relative rotation between pipe sections necessary to run threads to make up and break out threaded connections. Powered lead tongs have pipe rotating motors. Back-up tongs usually have power applied to closing the dies on pipe but usually do not have pipe rotating power.
Some tong sets are a close coupled pairing of lead and back-up tongs. The set then transmits torque between the tongs and no conventional external force acting on a torque arm is required on either tong. Ideally, the torque transmitted between tongs is pure torque. In that case, the torque alone would not produce a side load on pipe. The structure connecting the early tong sets was rather rigid, however, and could produce pure torque only if all gripping and gripped surfaces rotated true about the same axis. In the field of pipe string assembly, geometric perfection is rare and side loads on pipe can result from geometric irregularities. With geometric irregularities present, the more rigid the structure connecting the tongs the more severe is the resulting lateral loads.
The side loads became recognized as a complicating factor in torque measurement, and the side loads sometimes damaged the threads being mated. Tong sets with connecting structures arranged to apply pure torque to connections, with or without geometric irregularities, are in the art. One example of such structure is taught by U.S. Pat. No. 5,099,725 issued Mar. 31, 1992. Thread runs in either direction require some form of freedom of relative axial movement between tongs. In this example, there are two other degrees of freedom, or float, along perpendicular horizontal axes.
Spiders are the contrivances that are supported by the derrick floor structure, and support a suspended pipe string. The spider gains gripping power by applying at least part of the pipe string load to the mechanism that forces the dies onto the pipe surface. The flush design spider rests in, or at least occupies, the rotary table recess and is preferably flush with the derrick floor. Most spiders have fluid powered cylinders that open and close the pipe gripping dies, or slips. When the pipe load is large, the spiders designed to do so will grip the pipe rotationally and can function as a back-up tong. Spiders exist that have enough slip closing force provided by fluid power to allow the spider to function as a back-up tong without depending upon pipe load to close the slips.
The dynamics of a suspended length of pipe makes the final alignment of the length with the pipe string coupling near the center of the spider a very time consuming manual activity. The suspended pipe, approaching a vertical situation tends to oscillate laterally and immediate close control of such massive loads is beyond the realm of efficient manual handling. That particular aspect of pipe string handling is time consuming and dangerous.
A grabber is a machine assembly that extends an arm and pipe clutching means some distance toward an approaching pipe section, closes clutch jaws, or the "clutch", about the pipe and moves it toward the extended centerline of the tong to stab the connection.